The invention relates to a method for testing the tightness and correct closure of a plurality of containers which are transported on a conveyor, the containers succeeding each other at brief time intervals, the containers being sealed by attaching a closure, and a characteristic of the containers which is dependent on the internal pressure (=internal-pressure characteristic) being measured at time intervals after the attachment of the closure.
In the case of rigid containers such as glass bottles, the internal-pressure characteristic is typically the oscillation frequency of the closures, but in the case of flexible containers, such as PET bottles, it is the fill level.
Until now, the tightness of rigid containers, e.g. glass bottles which contain fruit juice or beer, has been tested by measuring the internal pressure. This test was carried out about 5 minutes after filling and closure. In the case of fruit juices which are poured in when hot, a negative pressure develops within this period due to cooling, whereas in the case of beer a slight positive pressure of 0.6 to 1.5 bar builds up due to the CO2 which is released. In the case of fruit juices which are heated in a pasteurizer after closure, a higher pressure develops. It is known to ascertain this pressure by measuring the oscillation frequency of the container closure (DE-A-40 04 965 and DE-A-196 46 685). The measurements are encumbered with great uncertainty, as the measured frequency is evidently affected by other parameters, regarding which reference is made to the simultaneously filed international patent application xe2x80x9cMethod for Testing Sealed Containersxe2x80x9d (=DE patent application 198 34 218.7).
The object of the invention is therefore, in the case of a method of the type described at the outset, to improve the reliability of the testing of the tightness or the closure.
According to the invention, this object is achieved in that:
during the attachment of the closures to the containers
the internal-pressure characteristic is measured and/or
parameters of the closures or containers are recorded, knowledge of which is necessary to ascertain the internal pressure from the measurement of the internal-pressure characteristic;
the containers are marked, no later than when the closure is attached, in a way which permits the values which are measured or recorded upon attachment of the closures to be allocated to the respective container,
the internal pressure of the container is ascertained form the value of the internal-pressure characteristic measured at the time interval after the attachment of the closure
and comparison with the value of the internal-pressure characteristic measured upon attachment of the closure, or
taking into account the recorded parameters.
A criterion for the tightness or correctness of the closure is derived from the ascertained internal pressure value. Of course, the internal pressure need not be measured numerically. It is sufficient to ascertain a variable which is representative of the internal pressure, or else only to establish whether this variable lies above or below an empirically ascertained threshold value.
The fact that the internal-pressure characteristic of the containers is recorded when attaching the closures means that this recording takes place before the internal pressure rises or falls. If a parameter of the closures or containers is recorded, knowledge of which is necessary for the measurement of the internal-pressure characteristic, it is sufficient to record this parameter, as long as the respective container can be tracked and an allocation is therefore still possible.
The period between attaching the closures and measuring the internal-pressure characteristic can be so great that a large number of containers can be transported on the conveyer within this period.
In the case of drinks containing CO2, the measurement takes place e.g. after 10 minutes. In the case of fruit juices which are pasteurized, the internal-pressure characteristic can be measured after the pasteurizer.
With the process alternative, in which the internal-pressure characteristic is recorded when attaching the closures to the containers, this characteristic is measured twice, namely the first time when attaching the closures to the containers and the second time after the time interval mentioned. The first measurement then corresponds to the internal pressure zero, so that this value essentially depends only on the properties of the closure and/or cap (=closure parameters). The deviation of the value obtained during the second measurement from the value obtained during the first measurement is therefore almost exclusively attributable to the change in the internal pressure, so that the difference in the measured value shows, directly and in general even linearly, the increase or reduction in the internal pressure.
If, on the other hand, the properties of the closure and/or the cap (=closure parameters) are recorded when attaching the closures to the containers, the internal pressure is then ascertained during the later measurement of the internal-pressure characteristic from the measured value of this characteristic, by means of empirical values which are stored in value tables for the parameters. Each cap and/or additionally each closure type then has, so to speak, its own parameter set and threshold value for the measured frequency in the case of rigid containers or of the fill level in the case of plastic containers, with which the ascertained frequency or the ascertained fill level are compared. The influence of the properties of the closure blank or attached closure, e.g. material thickness and compound quantity), and of the properties of the closure chuck, e.g. the closure force, on the oscillation frequency of the attached closure is described in the simultaneously filed international patent application xe2x80x9cMethod for Testing Sealed Containersxe2x80x9d (=DE patent application 198 34 218.7).
The marking of the containers makes possible a permanent allocation to the individual containers of the value of the characteristic recorded when attaching the closure or of the other parameters.
The measured values and the parameters can be attached direct to the container by means of the marking.
Another possibility consists of consecutively numbering the containers by means of the marking and then storing with each container number in a computer the internal-pressure characteristic measured during the attachment of the closure or the recorded closure parameters. The consecutive numbering can take place periodically as, by and large, the containers are moved along or emerge from the pasteurizer in turn. If the containers are already continuously or periodically numbered, this numbering can be used for the allocation of the recorded value or parameters to the containers.
In the case of rigid containers such as glass bottles, however, the marking preferably contains only information about the oscillation frequency of the closure as ascertained during the first measurement, the first measurement in general taking place directly after closure. As indicated, this oscillation frequency depends primarily on the type of closure and the closure force of the closure chuck (=closure parameters). At the second measurement, if e.g. a positive pressure has built up in a beer bottle sealed with a crown cap, a frequency deviating from this is measured, in most cases a higher frequency. The frequency increase is solely attributable to the positive pressure which has built up in the meantime. During the first measurement, carried out directly after the pressing on of the crown cap, the frequency is e.g. between 7 and 8 kHz, the differences between the individual bottles being caused by differences in the thickness of the crown cap material and the varying closure force of the closure chucks (closure parameter). After 5 minutes, a positive pressure e.g. of 1 bar has built up, which led to a frequency higher by 0.7 kHz being measured during the second measurement that was then carried out. This frequency increase lies within the fluctuation band which is caused by differences in the closure parameters. A frequency measurement only after the build-up of the internal pressure would therefore not permit a reliable statement regarding the tightness of the bottle closure; it would not be possible to distinguish with certainty between frequency deviations caused by closure parameter differences and those which can be attributed to an increase in the internal pressure.
The marking can take place in the form of a bar-code. In practice, this kind of marking can run into difficulties, as a bar-code would then have to be attached to the container or closure, something which, up to now, has not been accepted by consumers. The marking can also be attached with UV ink. The marking, i.e. the allocation of specific recorded values to the containers, can take place by means of a transponder.
Instead of using a bar-code, UV ink or transponder, the container or the closure can also be marked magnetically. This can take place via the container or closure material, e.g. using ferromagnetic material, or through the use of a magnetizable coating, similar to that in the case of computer diskettes. In principle, analog or digital operation is possible, i.e. via the strength of the magnetic field or only via the direction. The two processes can also be combined.
The magnetic marking can take place by three different methods.
a) Analog encryption of the information
b) Analog encryption of the information with reference marking
c) Digital encryption of the information
A disadvantage of the analog method is that effects which influence the intensity of the magnetization have to be monitored. These include e.g. the unavoidable fluctuations in material thickness and composition of the closure. This can be corrected by firstly impressing a write pulse of defined amplitude onto the top and reading it. The magnetizability of the top can then be deduced from the level of the read signal. On the basis of this information, the necessary amplitude of the write pulse can then be calculated for information storage. Another correction possibility consists of carrying out the magnetization simultaneously in several directions. For example, in the plane of the closure and perpendicular thereto. The magnetization is then read again in all three spatial directions and the magnetization in the plane of the closure, e.g. in the centre of the closure, is calculated from the magnetization in two of the three directions. The information then remains in the angle of the vector of the overall magnetic field strength above the plane of the closure. Therefore, no alignment of the bottle is necessary. The effects of the magnetizability of the individual closures are calculated from this.
Upon the analog encryption of the information, the height tolerances of the bottles are also to be borne in mind.
Possible Measures
With the process described above, in which firstly a write pulse of defined amplitude is impressed onto the closure, the height tolerance is corrected simultaneously. Height tolerances of the bottles and thus of the distance between the closure and the write and/or read head can also be actively measured and integrated into the evaluation of the magnetic field strength. Finally, the position of the write and read heads can be actively tracked to the bottle height.
These disadvantages of the analog method can also be dealt with by attaching a reference pulse of constant amplitude to the closure. This constant reference pulse is then used again to reconstruct the information in the variable write pulse. In this way, the problems described above are solved simultaneously.
The digital method naturally has a better signal-to-noise ratio, as only the direction of the magnetic field is used as an information unit. Therefore, different areas of the top have to be magnetized in different directions.
For example, the closure can be magnetized orthogonally to the closure surface plane in different orientations. Simple patterns for the magnetization are strips of different polarity or, in order to maintain the rotation symmetry, concentric rings of different polarity.
All these methods are possible. They differ only in the costliness of the process and in the amount of information which can be obtained and packed into the marking.
Finally, an increasing of the information density is also possible by combining the analog and digital processes. For example, concentric rings of differing orientation are applied to the cover, and these then also in several levels of magnetic field intensity which can be easily distinguished from one another. Every additional level then increases the maximum possible information quantity count (i.e. of the distinguishable states).
Both the magnetization and the reading of the closures are preferably independent of direction, i.e. the magnetization pattern is rotationally symmetrical to the rotation axis of the containers, e.g. of glass drink bottles with crown- or screw-caps. The closure can be magnetized in concentric rings, as already mentioned, for this purpose. Another possibility is to store the information in the direction of the magnetization. Because of the required independence from direction or rotational symmetry, an angular range of 180xc2x0 is all that is available overall.
As far as the design of the write head is concerned, a live coil, possibly with a highly permeable core, is already sufficient to write analog signals. The strength of the analog signal is regulated by the coil current. A field formation can be produced by suitable pole shoes on the soft magnetic core of the coil. If the rotational symmetry is to be maintained, it is simplest to apply the magnetization pulse perpendicularly to the plane of the top. The information is then included in the amount and in the sign of the magnetization. To this end, the coil is to be attached as a write head above the passing container and an electrical pulse of corresponding strength produced when a container is located under the write head.
A simultaneous magnetization in several directions can be realized by installing two or more coils which are triggered at the same time. Here too, magnetic field strength and magnetic field form can naturally be optimized by suitably shaped pole shoes.
Magnetization patterns in concentric rings on the closure can be realized by nested coils which have variable current direction and current.
The magnetic field of a live wire is sufficient to align electron spins of the closure material. For example, strip patterns can thus be applied to the closures by parallel live wires. Patterns comprising for example ten strips can be produced in this way. Rotationally symmetrical magnetization patterns can correspondingly be produced by annular wires.
Pointwise high magnetic field strengths, which can locally align the orientation of the electron spin of the material comprising the top can be produced by tips made of highly permeable material.
Depending on the complexity of the patterns, the read head consists of one or more magnetic field sensors. The information is reconstructed again by analysis software from their output signals. Hall sensors are sufficient for simple magnetization patterns. Magnetoresistive sensors are preferably used, being much more sensitive than Hall sensors. The considerably more expensive SQUIDS or the so-called GMRs (Giant Magnetic Resistivity Detectors) can also be used of course, but in general however, magnetoresistive sensors are sufficient and deliver a resolution sufficient for reading the magnetization patterns again, even at a distance of several millimeters. Their output signals deliver locally resolved information regarding the amount and direction of the measured magnetic field.
When magnetizing ferromagnetic materials, dependency on the previous history, that is the hysteresis, has to be considered. The flux density B which can be obtained in the material depends not only on the impressed outer magnetic field strength Hext but also on the previous history of the material.
For both the analog and, in a limited form, the digital method of magnetization, it is therefore advantageous to produce a defined initial magnetization for all closures in order to increase the reproducibility of the write pulses. This means that an additional erase head is also needed, upstream from the write head.
A saturation magnetization is one suitable defined initial magnetization and a demagnetization of the closures is another. Saturation magnetization is achieved by a very strong outer magnetic field. Demagnetization is achieved by means of a magnetic alternating field of decreasing intensity which is produced by means of a coil carrying alternating current. If the container closure is transported past the erase head, it automatically experiences a decreasing magnetic field, even if the coil current is constant. For demagnetization, an erase head is therefore sufficient, which is positioned a small distance above the container closures which are passing along beneath it and fed with alternating current of constant strength. The diameter of the erase head is larger than the diameter of the closure which is to be demagnetized in order to demagnetize the latter over all of its surface.
When writing, a similar problem arises if the write head is operating with coils which have soft-magnetic cores. After a write pulse of high field strength, a write pulse having the same direction cannot be produced which is smaller than the remanence belonging to the first pulse. To solve this problem, the following measures can be taken:
a) Only use the range between remanence and saturation magnetization as the dynamic range of the write head.
b) Between the bottles, provide an erase pulse for the write head by means of alternating current.
c) Pre-set, not the current, but the magnetic field of the write pulse. The magnetic field must then be measured and the currents of the write head set accordingly.
The more elegant process is naturally to integrate the write head and erase head in one head. The closure and the write head are thereby simultaneously de-magnetized. For this purpose, only a suitable write pulse need to be generated.